The present invention relates generally to a shipping container for semiconductor devices and more specifically a bakeable tape carrier for such devices.
Integrated circuit devices require a means for protective handling and transporting of the finished parts in order to avoid mechanical damage to the lead tips, the lead finishes, or assembled packages, as well as to provide environmental protection from moisture and from static charges. Further, the integrated circuit (IC) devices must be transported in carriers that are compatible with the customer""s in-house equipment system. For these reasons, the dimensions and design of primary carriers for integrated circuit devices have received considerable attention from worldwide committees, and have been standardized so that the using customer is not subjected to variations from different suppliers. Leaded surface mount devices, as well as more advanced area array packaged devices are transported in tape carriers on reels, while some devices are stored and transported in plastic trays with an array of recesses.
In addition to storing and transporting semiconductor devices in carriers, it is often desirable to xe2x80x9cdry bakexe2x80x9d the devices in the same carriers in order to minimize handling. Plastic molded surface mount integrated circuit devices are subject to delamination of the encapsulating plastic mold compound from the leadframe or substrate during solder attachment to a printed wiring board. In extreme cases the package cracks, which may in turn result in opens, or other reliability failures during use, or in testing. The failure mechanism has been traced to absorption of moisture by the molding compound, which vaporizes when the device is subjected to rapid heating during solder reflow processing, and the resulting vapor exerts pressure at the plastic to metal interface. This problem has intensified as packages have become larger and thinner.
In order to avoid such failure, devices which are subject to the problem must be xe2x80x9cdry bakedxe2x80x9d at a temperature high enough, and for sufficient length of time to drive the moisture out of the package. The devices undergo dry baking after they have completed assembly processing and testing, and again after the containers have been opened, if they have not been used within the allowable time.
Dry bake specifications require a time and temperature combination which will assure desorbtion of moisture prior to surface mount assembly. It is preferable to bake at the maximum temperature of 125 degrees C for shorter time, rather than an alternate dry bake processes with low temperatures such as 55 degrees C which require as much as 68 days to accomplish the objective. There are a number of obvious objections to the longer time including requiring more bake equipment, space, and utilities, as well as the cycle time increase. Standards committees, such as EIA (Electronic Industries Association) and JEDEC (Joint Electronic Devices Engineering Council) have set specific standards related to this issue based on integrated circuit packages type, standards for testing procedures, and also standards for packing containers used for the shipping and storage.
Tape carriers on reels provide a compact means for storing, transporting, and dispensing integrated circuits. The reel is placed directly onto a relatively small piece of equipment for picking and placing the circuits, and as a result this type of handling equipment has become more desirable to the end user than the more bulky x/y table used to pick and place from trays.
FIG. 1 illustrates a tape and reel carrier in which integrated circuit packaged devices 101 are held in a series of in-line recesses 102 in a carrier tape 103. The upper surface of the carrier tape is sealed by a cover tape 104 to hold the devices in place, and avoid contamination. Width of the tape is governed by size of IC packages. The tape is wound onto a reel 105. The reel diameter is kept constant for compatibility with equipment at both the user and supplier.
Unfortunately, while the details of carrier and tape design have been well defined by the industry, little attention has been placed on the thermal characteristics of the materials which would which maintain integrity of the carrier and cover tapes at maximum dry bake temperature of 125 degrees C.
A commonly used carrier tape is a formed polycarbonate polymer, and a typical cover tape material is a polyester polymer. The materials are not mutually compatible for high temperature applications, and in fact distortion of the cover tape occurs with resulting delamination from the carrier when the exposed to high temperature bake at 125 degrees C. Heat deflection temperature for PET polyesters is about 41 degrees C, and that of polycarbonate is about 140 degrees C, thus resulting in poor service of the polyester cover film for dry bake.
Existing adhesive technologies include pressure sensitive and heat sensitive compounds. The adhesive must hold cover tape in place without any delamination, but must release at a consistent strength throughout the tape length, and that pull force must be 130 grams force. Adhesive strength of heat sensitive adhesives typically increases by additional cross linking upon exposure to extended time/temperature excursions, thus making them unsuitable for dry bake processing.
A strong need exists in the industry for a robust, system of carrier and cover tape which is compatible with existing automation, and maintains integrity, during dry bake, and/or multiple heating and cooling cycles encountered during transportation, storage and uses of the integrated circuit devices prior to assembly on a printed circuit board.
It is an object of this invention to provide a carrier and cover tape assemblage for integrated circuit devices which will maintain integrity at exposure to temperature as high as 125 degrees Centigrade.
It is further an object of the invention that the carrier tape and cover tape assemblage be a simple replacement for the existing technology, requiring no change in processing equipment to the user.
It is also an object of the invention that the cover tape be transparent, so that the circuit devices are readily viewed.
It is an object of the invention to provide a carrier and cover tape combination which will release consistently upon demand at less than 130 grams peel force, after 125 degrees C dry bake for as much as 24 hours.
Another object of the current invention is to provide a carrier sufficiently robust to protect the IC devices from damage due to mechanical shock normally encountered during shipping.
Another object of the current invention is to provide a carrier system which has no appreciable change in weight and volume from existing carriers.
Yet another object of the invention is that it be sufficiently flexible to be assembled onto a reel.
The objectives of the current invention are met by providing a cover tape having the same thermal properties as that of the carrier tape itself. By employing the same material, and thus like thermal properties for both carrier and tape, the joining adhesive is placed under minimal stress to cause delamination, and subsequent exposure of the IC devices to the ambient.
The assemblage of the current invention includes a thin film polycarbonate cover tape with pressure sensitive adhesive laminated to a polycarbonate carrier. Polycarbonate polymer has a heat deflection temperature of about 135 degrees C, or a temperature well above that of the maximum dry bake temperature, and has been shown to be suitable for fabrication of carrier tapes and reels. In order to meet the objective of transparency, it is necessary that the cover tape be very thin, and be supplied without added colorant.
A pressure sensitive adhesive, known in the industry maintains consistent peel strength after prolonged thermal exposure at 125 degrees C. The peel force is less than 130 grams force, and thus is consistent with requirements of industry standards.
Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings. The drawings constitute a part of this specification and include an exemplary embodiment of the invention, which may be embodied in various forms.